It is the goal for many commercial applications to improve the quality of crystalline substrates, such as semiconductor substrates, and in particular compound semiconductors. Unlike materials such as silicon, which can be grown from the melt at relatively low cost in bulk single crystalline form, many compound semiconductors are fabricated by epitaxial growth of layers on single crystalline substrates. Often substrates such as silicon and sapphire are used to grow other single crystalline materials because of the high quality of the silicon or sapphire substrate. However, such heteroepitaxial processes still suffer from problems inherent in hetereoepitaxy.
In one important example, gallium nitride (GaN) is a material commonly grown on substrates for use in light-emitting diodes (LEDs). The ability to grow high-quality GaN is one limiting factor to improving the quality and lowering the cost of these devices. GaN growth on sapphire or silicon substrates typically exhibits crystal faults called threading dislocations or “threads.” These threading dislocations may be caused by the lattice size mismatches or thermal expansion mismatches between the GaN and the substrate. Threading dislocations can lead to poor efficiency, reliability, or lifetime in an LED due to poor conductivity. What is needed is an improved method of growing materials such as compound semiconductors and, more particularly, growing GaN. In particular, improved heteroepitaxy processes are needed.